"Us," says sociologist Sharon Harlan of Arizona State University (ASU). "It's all due to the effects of humans. We've modified the surface of the land in ways that retain heat."

Urban heat islands are the result. Soil and grass have been replaced with materials such as asphalt and concrete that absorb heat during the day and re-radiate it at night, causing higher temperatures.

Summer in a blistering desert

Harlan and colleagues in fields across the social, natural and health sciences are studying urban heat islands--and their opposites, park cool islands, where plant growth throws cold water on burning temperatures.

They're conducting the research via a National Science Foundation (NSF) Coupled Natural and Human Systems (CNH) grant. CNH is one of NSF's Science, Engineering and Education for Sustainability, or SEES, programs.

"Cities can be hot and uncomfortable places for the people who live in them, with some populations especially vulnerable to health problems from urban heat," says Sarah Ruth, CNH program director in NSF's Directorate for Geosciences.

"The city environment and its human inhabitants form a complex system with multiple connections. These researchers have uncovered important information about this system and the interactions among its components.

"The results suggest ways city officials and residents can work together to create places where fewer people suffer the effects of extreme temperatures."

The hot, arid Sonoran Desert is central Arizona's natural environment. Humans have transformed the desert over thousands of years, beginning with early Native American subsistence farmers and continuing with late 19th century Anglo-American commercial growers and 20th century sunbelt migrants.

They evaluated the effects of plants' cool greenness on a Phoenix inner-city park.

The results were recently published in the journal Urban Ecosystems. Along with Harlan, co-authors of the paper, all from ASU, are Juan Declet-Barreto, Anthony Brazel, Chris Martin and Winston Chow.

They're also working through the NSF Central Arizona-Phoenix Long-Term Ecological Research (LTER) site, one of 26 such NSF LTER sites across the nation and around the world.

"We predicted air and surface temperatures under two different vegetation regimes: Existing conditions representative of Phoenix urban core neighborhoods, and a scenario using principles of landscape design and architecture, and urban heat island mitigation strategies," write the scientists.

They found that the air beneath and around "canopied vegetation"--trees--was cooler than the surroundings.

Trees also reduce hot air by turning water from liquid to gas inside their leaves. "Temperatures then fall in the immediate environment," says Martin.

It all adds up to a park cool island. "Park cool islands are usually found in irregular patterns in a city," Declet-Barreto says. "They're nested within warmer spots."

The Latino Urban Core

The team studied the role of park cool islands in a low-income, ethnic minority community in inner-city Phoenix.

The area is called the Latino Urban Core. It's bounded by industrial land to the north, south and east and an interstate highway to the west. The neighborhood's main feature is an electric utility company easement currently used as "linear park space almost entirely devoid of vegetation," states the Urban Ecosystems paper.

The Latino Urban Core's sparse vegetation is mostly in residents' yards. Patches of exposed soil with nothing growing on them are scattered across vacant lots, yards and the grounds of the "linear park."

Parks in low-income neighborhoods tend to be hotter than parks in higher-income areas, research has shown. Although residents in low-income areas need places to cool off, these neighborhoods have less inherent cooling capacity as there's less green space.

Inner-city green space--lacking in Phoenix's Latino Urban Core--is a crucial component of urban heat island mitigation, Declet-Barreto says. "But it's made more difficult by ongoing debates over urban amenities like parks and the needed resources, such as water, tax dollars, local government will and regular maintenance."

Studies have shown that in Phoenix, inner-city areas bear higher property tax burdens in comparison with suburbs, but the former receive significantly fewer tax dollars for parks, recreation and water supplies.

Minority and low-income communities are increasingly addressing such disparities by demanding a more equitable distribution of urban amenities, such as green spaces like parks.

In low-income communities, parks are often the only available public gathering places. Green spaces, scientists say, can provide cultural, social and--more directly applicable to extreme heat mitigation--human health and ecological benefits.

Cooling down urban heat islands

Finding ways to offset high temperatures in desert cities where the weather is chronically hot, says Harlan, is critical.

Extreme heat, scientists have found, is a threat to human health, increases atmospheric pollutants and energy and water use, alters regional hydrology, and affects interactions between humans and ecological processes.

"The problem of heat-related deaths and illnesses is very serious," says Harlan. "Each year, heat fatalities in the U.S. happen in greater numbers than mortality from any other type of weather disaster." High heat wave events--unexpected and long-duration heat waves--are becoming more common in cities like Phoenix, Chicago and Paris.

Climate change and rapidly growing cities are likely to fuel more such events.

"Our research suggests that climate intervention strategies should be targeted at the neighborhoods and population groups that are most vulnerable to environmental hazards like extreme heat events," says Harlan. "We hope our results will be used in better decision-making about climate adaptation in cities."

Greening parks is an intervention strategy, she says, for urban heat island mitigation that could be supported with public resources.

"If targeted to low-income neighborhoods where vulnerability to heat is greater," says Harlan, "it would address an environmental inequity and provide better ecosystem services for these neighborhoods."

One antidote to an urban heat island, it turns out, is another island, a place filled with shade trees and lush growth: a park cool island.